The present invention relates to an atomic absorption spectrophotometer, and in particular to an atomic absorption spectrophotometer having an improved emission spectral distribution of a light source.
FIG. 1 shows an example of an atomic absorption spectrophotometer of the prior art. A light ray 10 emitted by a light source 1 passes through an acetylene flame 2 and enters a spectroscope 4. The spectroscope 4 selects the wavelength of the atomic absorption line of the element to be analyzed. Only the light ray having this wavelength is received by a photoelectric conversion element 5. The photoelectric conversion element 5 is connected to an amplifier 6, which in turn is connected to a logarithmic conversion element 7. The output of the logarithmic conversion element 7 is supplied to an output indicator 8 to indicate the electric signal thus converted logarithmically. An analysis sample 3 of a water solution is turned into a spray with air by a nebulizer 9 and led into an acetylene flame 2. This spray is evaporated in the acetylene flame 2. Inorganic substances dissolved in the analysis sample of the water solution 3 are thermally resolved in the acetylene flame 2 of approximately 4200.degree. C. to generate atom vapor. When the element to be analyzed is calcium (Ca), for example, a lamp such as a hollow-cathode lamp emitting a light ray of 422.67 nm in wavelength, which is the atomic spectral line of calcium is used as the light source 1. The wavelength of the absorption line of the atom to be analyzed which is generated in the acetylene flame 2 is also 422.67 nm. Matching in wavelength causes so-called resonance atomic absorption phenomenon. When the light ray 10 emitted from the light source 1 passes through the acetylene flame 2, therefore, the light having the wavelength of 422.67 nm is absorbed by calcium atoms existing in the acetylene flame 2.
Principle of Measurement
Assuming that
I.sub.0 =intensity of light (422.67 nm) of calcium atoms emitted from the light source PA1 I=intensity of light of calcium atoms after passing through the acetylene flame PA1 n=density of calcium atoms contained in the acetylene flame (1/cm.sup.3) PA1 l=length of the acetylene flame that the light passes (cm) PA1 k=absorption coefficient of calcium atoms, PA1 (1) kind of gas; PA1 (2) pressure of gas; and PA1 (3) temperature of gas
the well known Beer's law holds true as EQU I=I.sub.0 .multidot.exp [-n.multidot.n.multidot.l.multidot.k](1)
If the value of the ratio (I/I.sub.0) between electric signals varying in proportion to two light intensities is converted into a logarithm value, the value of [-n.multidot.l.multidot.k] which is in proportion to the density n of the atoms to be analyzed is obtained as evident from equation (1). Since the density n of the atom in the acetylene flame is in proportion to the concentration (ppm) of the element to be analyzed in the analysis sample of water solution, it is eventually possible to derive the concentration (ppm) of calcium contained in the analysis sample of the water solution 3. The concentration is the final object of the measurement. In the apparatus of the prior art, however, the principle of deriving the concentration of the aimed element is based on the photometry method using the well known Beer's law expressed by equation (1). In equation (1), light intensities I.sub.0 and I as well as the absorption coefficient k are functions of the wavenumber .nu. cm.sup.-1 of the light. Therefore, equation (1) can be rewritten as EQU I(.nu.)=I.sub.0 (.nu.).multidot.exp [-n.multidot.l.multidot.k(.nu.)].(2)
Since the wavenumber (.nu. cm.sup.-1) can be related to the wavelength .lambda. (cm) as EQU .lambda..multidot..nu.=1,
equation (2) can also be represented as a function of the wavelength. When it is defined that n.multidot.l.multidot.k=A, A is referred to as the "absorbance".
FIG. 2 shows the spectral distribution of incident light spectra I.sub.0 (.nu.) and absorption spectra k(.nu.) for the atomic spectra of calcium (Ca). The ordinate of FIG. 2 represents the values of the functions I.sub.0 (.nu.) and k(.nu.). The abscissa represents the wavenumber .nu.(cm.sup.-1). Curve 3' and 4' represent the spectral distribution of the functions k(.nu.) and I.sub.0 (.nu.), respectively. The wavenumber .nu.(cm.sup.-1) whereat the function k(.nu.) assumes the maximum value is shifted from that of the function I.sub.0 (.nu.) by .DELTA..nu.(cm.sup.-1). As described in Wagenaar et al. "Spectrochimina Acta", Vol. 28B, Pages 157-173, 1972, this shift value depends upon the element and in general ranges from 0.01 to 0.08 cm.sup.-1. For calcium, this shift is 0.037 cm.sup.-1.
As well known by the above described literature written by Wagenaar et al., for example, the shift between the wavenumber whereat the emission spectrum I.sub.0 (.nu.) is maximized and the wavenumber whereat the absorption spectrum k(.nu.) of the same atom is maximized depends upon:
wherein the calcium atoms are disposed. When the temperature and pressure of the atmospheric gas wherein the atoms are disposed are raised, the maximum values of I.sub.0 (.nu.) and k(.nu.) are displaced in such a direction as to decrease the wavenumber .nu. cm.sup.-1.
Table 1 shows the difference in atmospheric gases wherein the calcium atoms are disposed in the apparatus of the prior art as illustrated in FIG. 1.
TABLE 1 ______________________________________ Difference between atmospheric gas of light absorption atoms of calcium and that of emission atoms Temper- Spectral Kind of Pressure ature No. Atmosphere function gas of gas of gas ______________________________________ 1 Hollow- k(.nu.) Neon 8 Torr. 600.degree. K. cathode lamp 2 Acetylene- I.sub.0 (.nu.) Acetylene 760 Torr. 2400.degree. K. air flame air ______________________________________
Table 1 shows the case of calcium atoms. In general, the gas enclosed in the hollow-cathode lamp of the prior art apparatus is neon (Ne) or argon (Ar), and its pressure ranges from 4 to 10 Torr. The temperature at the hollow cathode portion is raised as the discharge current (2 to 40 mA) is increased, and is in the range of 400.degree. to 1200.degree. K. The acetylene flame having absorption atoms therein is under the atmospheric pressure (760 Torr.). The temperature of the acetylene flame varies somewhat depending upon the flux of the air mixed with acetylene, and the temperature is in the range from 2200.degree. to 2700.degree. K. In every kind of analyzed element in the prior art apparatus of FIG. 1, the position on the axis of the wavenumber .nu. cm.sup.-1 whereat the emission spectrum I.sub.0 (.nu.) of the light source 1 is maximized is always shifted in such a direction as to increase the wavenumber with respect to the position on the wavenumber axis whereat the absorption spectrum k(.nu.) of atoms in the acetylene flame 2 is maximized. This shift is in the range of approximately 0.01 to 0.08 cm.sup.-1 This fact is also reported in the above described literature written by Wagenaar et al. in detail.
If the central wavenumber (.nu.) of the spectral distribution of I.sub.0 (.nu.) does not match with that of k(.nu.) in equation (2), the absorption phenomenon appears in a section on the wavenumber axis wherein the emission spectrum and the absorption spectrum overlap each other. When the density of the atoms to be analyzed is low, absorption proportioned to the density is conducted on the overlapped wavenumber section. When the density becomes high, however, the emission spectra belonging to the overlapped wavenumber section are sufficiently absorbed to approach zero. Even if the density of the atoms to be analyzed is further raised, therefore, the amount of absorption no longer changes. Accordingly, a linear relationship does not hold true between the electrical signal -Log [I(.nu.)/I.sub.0 (.nu.)]=Log [I.sub.0 (.nu.)/I(.nu.)] derived from the apparatus of FIG. 1 and the concentration C of the element to be analyzed. FIG. 3 shows a working curve 5' for the atomic absorption line (422.67 nm) of calcium (Ca) derived by using the prior art apparatus of FIG. 1. The working curve is a plot of the electrical signal Log [I.sub.0 (.nu.)/I(.nu.)] (ordinate) as a function of the concentration C ppm of the element to be analyzed (abscissa). As evident from FIG. 3, the working curve 5' is warped as the concentration C ppm is increased. If the concentration C exceeds 80 ppm, the electrical signal Log [I.sub.0 (.nu.)/I(.nu.)] scarcely increases. That is to say, the precision in measuring the concentration C is significantly lowered, the practical measurement being made impossible. This curve of the working curve depends upon the area of the overlapped wavenumber section between the emission spectrum and the absorption spectrum. Thus, the atomic absorption spectrophotometer of the prior art such as an example illustrated in FIG. 1 has a drawback that the working curve for every element to be analyzed is so warped toward the horizontal direction as to make measurement impossible for a concentration C ppm exceeding a certain value as illustrated in FIG. 3. Such a warp of the working curve is described in "Atomic absorption analysis" by Yasuda et al., pages 22-33, 1972.
If a magnetic field is applied to the light source, the emission spectrum line is separated according to the magnitude of the magnetic field and a wavenumber shift is caused. That is to say, the so-called Zeeman effect is caused as described in Harvey Elliot White, "INTRODUCTION TO ATOMIC SPECTRA", page 152, 1968, for example.
An attempt to raise the sensitivity of the atomic absorption by using the Zeeman effect is described in Schrenk et al., "SPECTROSCOPY LETTERS", 1(6), pages 237-244, 1968. That is to say, a magnetic field of 3400 Gauss is applied to a light source composed of a hollow-cathode lamp of iron Fe in a direction perpendicular to the direction of light emitted from the light source. This results in the raised sensitivity of the atomic absorption lines of elements to be analyzed other than Fe, i.e., manganese Mn, nickel Ni, and copper Cu. When this method is used, the radiation atomic spectrum of ion is disrupted into three components .sigma..sup.+, .sigma..sup.- and .pi. by the Zeeman effect. Since all of these three components are used, the wave number whereat the emission spectral distribution I.sub.0 (.pi.) is maximized does not coincide with the wave number whereat the absorption spectral distribution k.sub.0 (.nu.). In addition, the linearity of the working curve has not been considered at all.